Two reasons:

I think seeing a ball go over the line is, in almost all cases, a much easier job than determining if a player is offside. For the first, you only have to look at the ball to see if it crosses the line. Offside requires a linesman to simultaneously look at the passer and the receiver to see when the ball leaves the foot and where the receiver is at that moment.

The more important reason in my eyes is that the first would be so simple to remove from the game by adding some form of goal line technology. A sensor in the ball will work just fine. I think people get so worked up about missed goal line calls is because they are so damn easy to remove from the game without having any adverse effects on the play. As far as I know, offside "technology" does not exist. There is of course instant replay, but I honestly don't know if that would work very well in football. Some say it would be fine, but I've seen so many times where play continuing has led to an advantage for a team. If play were stopped all those times for replays, it would definitely have an effect on the game.